1. Field of the Invention (Technical Field)
The present invention relates to circuits for generating high voltage electrical pulses.
2. Background Art
With the thrust to maintain a clean environment, the need for better particulate control in industrial processes is needed. Electrostatic precipitators are one of the most widely used methods of collecting particulate matter in flue gas systems. In general, the systems are comprised of sets of collecting plates which are usually at ground potential, high voltage electrodes, and a set of power supplies which delivers the high voltage to the electrodes. The high voltage electrode is made up of either a thin wire running the length of the collecting plates or a rigid electrode.
The majority of the power supplies for the systems are made up of transformer/rectifier (T/R) sets. The T/R sets provide unfiltered, rectified high voltage (40 kV-80 kV) DC to the electrodes. It has been shown that better collection efficiency can be achieved by applying a voltage pulse (also known as pulsed energization) to the electrostatic precipitator instead of the unfiltered DC.
The idea of pulsed energization for the electrostatic precipitator (ESP) process is not new and has been studied extensively. The earliest work was performed by R. Heinrich in the 1920""s. Heinrich applied radar modulator technology that was being developed for World War II. Heinrich worked with Harry White and Herb Hall at the MIT Radiation Laboratory. Later in 1952 the first full scale tests were conducted by White and Hall when they applied pulsed voltages with 50 microsecond rise-times to wire-plate ESPs using rotary spark gaps or hydrogen thyratrons, pulse transformers and blocking diodes. Improved efficiency was observed for the tests.
Masuda of Japan furthered pulsing technology in 1976 by applying a pulse to a DC bias. The bias allowed the use of halo-wave AC for corona creation. This system operated primarily as a pre-charger to a conventional ESP. In the United States numerous systems have been investigated over the years with limited success due to overall system costs and reliability.
Prior art patents that disclose related technology, however different from the present invention, include: U.S. Pat. No. 5,623,171, to Nakajima, entitled xe2x80x9cHigh Voltage Pulse Generating Circuit and Electrostatic Precipitator Containing It;xe2x80x9d U.S. Pat. No. 4,808,200, to Dallhammer et al., entitled xe2x80x9cElectrostatic Precipitator Power Supply;xe2x80x9d U.S. Pat. No. 4,558,404, to James, entitled xe2x80x9cElectrostatic Precipitators;xe2x80x9d U.S. Pat. No. 4,867,765, to Tomimatsu et al., entitled xe2x80x9cSelf-Discharge Type Pulse Charging Electrostatic Precipitator;xe2x80x9d U.S. Pat. No. 4,600,411, to Santamaria, entitled xe2x80x9cPulsed Power Supply for an Electrostatic Precipitator;xe2x80x9d U.S. Pat. No. 4,592,763, to Dietz et al., entitled xe2x80x9cMethod and Apparatus for Ramped Pulsed Burst Powering of Electrostatic Precipitators;xe2x80x9d U.S. Pat. No. 2,509,548, to White, entitled xe2x80x9cEnergizing Electrical Precipitator;xe2x80x9d U.S. Pat. No. 5,903,450, to Johnson et al., entitled xe2x80x9cElectrostatic Precipitator Power Supply Circuit Having a T-Filter and Pi-Filter;xe2x80x9d U.S. Pat. No. 5,757,169, to Terai, entitled xe2x80x9cElectric Circuit for Pulse Energized Electrostatic Precipitator and Pulse Energized Electrostatic Precipitator Using This Circuit;xe2x80x9d U.S. Pat. No. 5,639,294, to Ranstad, entitled xe2x80x9cMethod for Controlling the Power Supply to an Electrostatic Precipitator;xe2x80x9d U.S. Pat. No. 4,996,471, to Gallo, entitled xe2x80x9cController for an Electrostatic Precipitator;xe2x80x9d U.S. Pat. No. 4,626,261, to Jorgensen, entitled xe2x80x9cMethod of Controlling Intermittent Voltage Supply to an Electrostatic Precipitator;xe2x80x9d U.S. Pat. No. 4,587,475, to Finney, Jr. et al., entitled xe2x80x9cModulated Power Supply for an Electrostatic Precipitator;xe2x80x9d U.S. Pat. No. 4,567,541, to Terai, entitled xe2x80x9cElectric Power Source for use in Electrostatic Precipitator;xe2x80x9d U.S. Pat. No. 4,541,848, to Masuda, entitled xe2x80x9cPulse Power Supply for Generating Extremely Short Pulse High Voltages;xe2x80x9d U.S. Pat. No. 4,290,003, to Lanese, entitled xe2x80x9cHigh Voltage Control of an Electrostatic Precipitator System;xe2x80x9d and U.S. Pat. No. 4,061,961, to Baker, entitled xe2x80x9cCircuit for Controlling the Duty Cycle of an Electrostatic Precipitator Power Supply.xe2x80x9d
U.S. Pat. No. 4,558,404, to James, relates to a DC-AC inverter, whereas the present invention utilizes an inversion circuit which inverts polarity. U.S. Pat. No. 4,867,765, to Tomimatsu et al., uses an inductive isolation for the voltage pulse, while other ESP cells are used to bleed down the charge on the ESP. Only one ESP cell is pulsed at a time due to the multi-pole output switch. U.S. Pat. Nos. 4,808,200, to Dallhammer, et al. and 4,567,541, to Terai, both use a voltage pulse which is superimposed on top of a constant DC value, which is not the case in the present invention. U.S. Pat. No. 4,600,411, to Santamaria, uses a pulsed source and a transformer, as well as an LC trap circuit which is quite different from the present invention. U.S. Pat. No. 4,592,763, to Dietz, et al., discusses a pule ramping technique which is not relevant to the present invention. U.S. Pat. No. 2,509,548, to White, discloses the use of a step-up transformer or a MARX arrangement to generate high voltage output. Again, this is different from the present invention. U.S. Pat. No. 5,903,450, to Johnson et al., is an invention for an inductive PI filter on the output of a transformer/rectifier set and is not relevant to the present invention. U.S. Pat. No. 5,757,169, to Terai, discloses an energy recovery technique that is quite different from the present invention. U.S. Pat. No. 5,639,294, to Ranstad, is an invention for a short circuit protection circuit for the electrostatic precipitator. U.S. Pat. Nos. 4,996,471, 4,626,261, 4,290,003, and 4,061,961, all disclose control circuits which are unrelated to the present invention.
The present invention is of a system which improves the efficiency of ESP performance which can be used for high resistivity ash collection. The technology is based on economically converting existing capital equipment such as existing T/R sets to produce optimized power delivery to the ESP load. A power supply can alternatively be used.
Performance improvements are gained through the ability of the system to operate at higher average currents and voltages. It has been shown that if ionization potential is exceeded quickly enough, a uniform corona distribution is obtained. This leads to uniform current distribution and improved ionization of particles. The present invention is different from other pulsed energization schemes discussed earlier because it does not force fast rise-time,  less than 1 microsecond, or square pulse shapes. In fact, the concept provides a moderate pulse rise-time in the range of 1 microsecond to 500 microseconds and then allows the voltage to decay down naturally dependent on the ESP load characteristics.
The present invention is of a slow pulse generating circuit for generating slow rise-time, high voltage electrical pulses to a load. The circuit comprises means for producing a pulsed voltage, means for charging the means for producing pulsed voltage, an energy recovery circuit for returning unused energy from the load back to the means for producing the pulsed voltage, a load matching circuit, means for inhibiting load voltage discharge back through the circuit, and means for transferring energy from the means for producing pulsed voltage to the load matching circuit. The circuit can optionally have a transformer for stepping up voltage from the means for producing the pulsed voltage to the load matching circuit. The means for producing the pulsed voltage preferably comprises either an inversion circuit or a high voltage switching circuit. The inversion circuit comprises at least one storage capacitor that is charged by the means for charging, a primary switch that is closed when the at least one storage capacitor becomes charged, and an inductor in series with the primary switch. The high voltage switching circuit comprises a primary switch, an inductor in series with the primary switch, and at least one storage capacitor for triggering the primary switch, and is located in series between the inductor and the at least one magnetic switch stage.
The means for charging the means for producing pulsed voltage preferably comprises a transformer/rectifier set or a power supply. The energy recovery circuit comprises at least one energy recovery diode and an energy recovery inductor in series with the at least one diode, and is located between the transformer/rectifier set (or power supply) and the load matching circuit. The energy recovery circuit also preferably comprises at least one energy recovery storage capacitor at the output of the transformer/rectifier set or power supply, at least one series charge element at the output of the T/R set or power supplyxe2x80x94which can be either a series charge resistor or an inductorxe2x80x94and at least one energy recovery diode to recover energy due to voltage reversals occurring in the load matching circuit.
The load matching circuit preferably comprises at least one load matching blocking diode and at least one load matching capacitor for charging the load. The means for inhibiting load voltage discharge preferably comprises at least one blocking diode. The means for transferring energy preferably comprises at least one magnetic switch stage. The at least one magnetic switch stage of the circuit preferably comprises at least one magnetic switch, and at least one capacitor to saturate each of the at least one magnetic switch.
The circuit preferably comprises a fire-on-voltage controller for determining the trigger voltage for the means used to produce the pulsed voltage. In an alternative embodiment, the circuit comprises means for producing pulsed voltage, means for charging the means for producing pulsed voltage, at least one blocking diode to inhibit load voltage discharge back through the circuit, and at least one magnetic switch stage for transferring energy from the means for producing pulsed voltage to the load. The means for producing pulsed voltage preferably comprises at least one storage capacitor to be charged to a preset voltage and a primary switch to be closed when the at least one storage capacitor becomes charged to the preset voltage. In this alternative embodiment, a fire-on voltage controller is preferably used for determining the preset voltage for the storage capacitor and then triggers the primary switch. This embodiment preferably further comprises an inductor in series with the means for producing pulsed voltage and at least one capacitor to transfer energy from the means for producing pulsed voltage to the at least one magnetic switch stage and load.
The present invention is adaptable to existing electrostatic precipitator systems having a high voltage power source, such as a transformer/rectifier set or power supply. The improvement over the existing electrostatic precipitator system comprises means for producing pulsed voltage connected to the power source, energy recovery circuitry for returning unused energy from the electrostatic precipitator back to the means for producing pulsed voltage, a load matching circuit connected between the means for producing pulsed voltage and the electrostatic precipitator, means for inhibiting the electrostatic precipitator load voltage discharge back through the system, and means for transferring energy from the means for producing pulsed voltage to the load matching circuit.
The present invention is also of a method of generating slow rise-time, high voltage electrical pulses to a load and comprises the steps of producing pulsed voltage, transferring energy from the means for producing pulsed voltage to the load with at least one magnetic switch stage, matching the load to the means for producing pulsed voltage, blocking the load voltage to inhibit discharge back through the circuit, and recovering and returning unused energy from the load back to the means for producing pulsed voltage. The method preferably further comprises the step of stepping up voltage from the means for producing pulsed voltage to the load with a transformer. The pulsed voltage is preferably produced from either a transformer/rectifier set or a power supply.
The step of producing pulsed voltage comprises the steps of charging at least one inversion circuit storage capacitor with charging means and closing a primary switch when the at least one inversion circuit storage capacitor becomes charged. Alternatively, the method of producing pulsed voltage comprises the steps of charging at lease one high voltage switching circuit storage capacitor located in series between a switching circuit inductor and at least one magnetic switch stage, and closing a primary switch when the at least one high voltage switching circuit storage capacitor becomes charged.
The method of recovering and returning unused energy from the load can be accomplished with at least one energy recovery diode and an energy recovery inductor in series with the at least one energy recovery diode. The step of recovering and returning unused energy from the load preferably further comprises the steps of recovering unused energy from the load with at least one energy recovery storage capacitor and at least one series charge element, both placed at the output of the means for charging the means for producing pulsed voltage, and recovering energy due to voltage reversals occurring in the load matching circuit with at least one energy recovery diode.
The step of matching the load to the means for producing pulsed voltage preferably comprises matching the load with at least one load matching blocking diode and at least one load matching capacitor that charges the load. The step of transferring energy from the means for producing pulsed voltage to the load preferably comprises the steps of saturating at least one magnetic switch stage with a charged capacitor and transferring energy from the means for producing pulsed voltage to the load with the at least one magnetic switch stage. Preferably, a fire-on voltage controller is used to control the trigger voltage for producing pulsed voltage.
The present invention is also a method of generating slow rise-time high voltage electrical pulses to a load comprising the steps of producing pulsed voltage, charging the means for producing pulsed voltage, blocking the load voltage to inhibit discharge back through the pulse generating circuit, and transferring energy from the means for producing pulsed voltage to the load with at least one magnetic switch stage. The step of producing pulsed voltage comprises the steps of charging at least one storage capacitor to a preset voltage and closing a primary switch when the at least one storage capacitor becomes charged to the preset voltage. The step of producing pulsed voltage also preferably comprises controlling the trigger voltage for closing the primary switch with a fire-on voltage controller.
A primary object of the present invention is to provide a slower rise-time for gradually getting energy to an electrostatic precipitator load without high current.
A primary advantage of the present invention is that a voltage pulse is produced on the electrostatic precipitator load, having a slower rise-time, and which aids in eliminating back-corona on the ESP plates.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.